With the development of information processing technique, a magnetic disk apparatus used as an external recording apparatus or the like in a computer is required to have a large capacity and a high performance such as fast transfer. In view of such a requirement, in order to achieve high recording density in magnetic recording, in recent years, a perpendicular magnetic recording system suitable for high density recording in principle is put into practical use.
Also in the perpendicular magnetic recording system, as in the case of a horizontal magnetic recording system, to reduce noise in a recording layer (or a magnetic layer) of a magnetic recording medium is effective in increasing recording density. Conventionally, a coercive force of the recording layer is increased, or magnetic particles composing the recording layer is miniaturized and isolated, whereby noise is reduced.
In order to reduce noise, so-called a granular layer composed of magnetic particles in a part of a perpendicular recording layer and a non-magnetic oxide or a non-magnetic nitride is used. When the recording layer is composed of the granular layer, an oxide or a nitride segregates around the magnetic particles, and magnetic separation between the magnetic particles is enhanced. It is relatively effective to provide a structure in which a ruthenium (Ru) underlayer is provided below the recording layer. The Ru underlayer is provided for the purpose of facilitating the magnetic separation between the magnetic particles composing the recording layer. With regard to the Ru underlayer, there is proposed a method for separating an Ru layer, to be provided directly under the recording layer composed of the granular layer, by a space (for example, Japanese Patent Application Laid-Open No. 2005-353256).
However, even if the recording layer is composed of the granular layer or has a structure in which the Ru underlayer is provided below the recording layer, there is a problem that it is difficult to further enhance a signal-to-noise ratio (SNR), a cumulative square error (VMM) as an indicator of an error rate, and rear/write properties of a magnetic recording medium represented by an effective track width WCw or the like. It is considered to be due to insufficiency of magnetic separation between the magnetic particles composing the recording layer.
The effective track width WCw is an effective width of a track. A write width of a magnetic head is measured from a profile of the result that data is written and read while offsetting the magnetic head to a track on a magnetic recording medium, whereby the effective track width is obtained.